Construction machines of this kind are known, in particular, in road construction operations during which road finishers are used to construct asphalt highways. These construction machines can, in principle, be equipped to run on wheels, caterpillar tracks, or a combination of wheels and tracks.
In the case of a tandem axle unit, also referred to as a bogie or pendulum axle unit, two axles are mounted one behind the other on a suspended and pivoted twin axle carrier. In this way the wheels can absorb unevenness of the surface of the ground such that the unevenness has no influence on the chassis. Twin axle carriers thus provide a means of stabilizing the machine as it travels, with the absorbing action of each twin axle carrier being possible independently of that of any other twin axle carrier.
These construction machines are usually in themselves very heavy. Furthermore, they are equipped with large-volume containers for accommodating construction material, with the result that the wheel axles must bear heavy loads. Apart from absorbing ground irregularities, tandem axle units have the advantage that they allow for displacement of the center of gravity of the machine and thus make it possible to alter the effective distribution of the machine load on its respective front and rear axles. If the pendulum axle of the tandem axle unit forms the front axle of the vehicle, and the separate wheels are disposed on the rear axle, as is the case in a road finisher, it is possible, for example, to move the center of gravity of the machine forward by raising the front wheels of the tandem axle unit so as to reduce the effective machine load on the rear axle. Particularly, when these machines are being driven, such a change in load distribution on the axles is advantageous, since it makes it possible to keep to the maximum permitted axle loads when the machine is driven on public highways. When used at a construction site, it can again be advantageous to place particularly heavy axle loads on the front or rear axles in order, for example, to increase the effectiveness of a screed plate disposed on a rear wheel.
A road finisher having two tandem axle units is described in DE 29920556 U1, in which case the pendulum axles are mounted directly on the chassis. In order to achieve a reduction in axle load on the rear axles by moving the center of gravity of the road finisher, one embodiment includes a regulating device, for example, a hydraulic cylinder, which acts on a supporting arm of the twin axle carrier. The regulating device is thus either subject to heavy strains when cooperating with the twin axle carrier or restricts the pendular motion mode to the extent that no further free pendular motion is guaranteed.
DE 29920556 U1 discloses, furthermore, an arrangement of the tandem axle units such that they can be moved longitudinally along the machine, likewise allowing for displacement of the center of gravity of the machine. Such a construction is, however, technically very demanding and thus expensive to produce. In addition, displacement of the twin axle carrier or related parts thereof requires a lot of physical space, and this has a negative influence on the basic dimensions of the machine.
In addition to the displacement of the center of gravity of the machine as determined by operating conditions, it is furthermore of interest to ascertain the effective axle loads of the machine, and to design the chassis in such a way so as to make it vertically adjustable, in order to optimize the construction process. A vertically adjustable road finisher makes it possible, for example, to align it exactly with trucks supplying construction material or removing excavated material. The combination of a vertically adjustable chassis with a freely pendular twin axle carrier, while allowing for the required displacement of the center of gravity, has hitherto not or only to an unsatisfactory degree been accomplished.